In motor control circuits it is often desirable to include amplifiers and comparators which operate over signal ranges from below ground up to the motor supply voltage (V.sub.CC), and in some instances even above V.sub.CC. These so-called "rail-to-rail" circuits are difficult to design and often require a number of design compromises.
One of the most difficult circuit design challenges, for example, is to build an amplifier that operates all the way from rail-to-rail. Amplifiers generally include current sources, which themselves subtract from the voltage available to the amplifier. If a rail below ground is available, however, a wide input range can be achieved without resorting to rail-to-rail construction.
A known method of solving this problem is to provide an additional voltage supply which generates supply rails below ground or above V.sub.CC. These additional voltage supplies generally add cost and complexity to the circuit because they require the integration of a charge pump or, in some cases, a switching mode power supply and an external inductor.
The present invention overcomes these problems.